System of converting direct current



8- 1939- R. H. SULLIVAN 2,171,279

SYSTEM OF CONVERTING DIRECT CURRENT Filed June 10, 1938 Patented Aug. 29, 1939 UNITED STATES 2,171,279 SYSTEM or CONVERTING nmnc'r CURRENT Raymond B. Sullivan, Rochester, N. Y., assignor to General Motors Corporation, Detroit, Mich, a corporation oi-Delaware Application June 10, 1938, Serial No. 212,963

1 Claim.

This invention relates to a system of converting direct current from low voltage to higher voltage, the output current being either direct or alternating, such system employing a vibratory 5 interrupter operated by an electromagnet from the source of direct current. When a vibratory interrupter is used to control the circuits of the system, the eiectromagnet which operates the interrupter is controlled by contacts, one of which is actuated by the interrupter. The life of an interrupter is largely dependent upon the life of the contacts which control the vibrator magnet.

It is an object of the present invention to prolong the life and improve the operation of a vi-' bratory interrupter for a D. C. transformer system by minimizing sparking at the contacts which control the electromagnet which operates the interrupter. In the disclosed embodiment of go the present invention, the suppression of sparks at the magnet controlling contacts is obtained by a resistance in shunt with the electromagnet coil.

Further objects and advantages of the present invention will be apparent from the following dea scription, reference being had to the accompanying drawing wherein a preferred embodiment of the present invention is clearly shown.

In the drawing:

A figure of the drawing is a wiring diagram 0 showing a D. C. converter system embodying the present invention.

The source of direct current is the storage battery 20. The transformer, indicated at 2|, comprises a core 22, primary windings 23 and 24 and 35 secondary windings 25 and 26. The primary windings 23 and 24 are centrally tapped and connected by a wire 21 with the battery. The outside leads of the primary windings are connected by wires 29 and 30 respectively with stationary 40 contacts 3| and 32 respectively of the vibratory circuit interrupter.

The vibratory circuit interrupter or vibrator comprises a flexible reed 33 fixed in a block 34 which is connected by a switch 35 to the battery 46 20. Reed 33 carries movable contacts 3|a and 32 and to separate the contact 42 from the conwire 39 and wire 6| with wire 21 and by wire 43 'with a stationary contact 4| which is normally engaged by a contact42 fixed to the reed 33 by a leaf spring 43. The contacts 4| and 42 constitutethe magnet controlling contact C. The 10 letter R. is applied to the magnet coil 38 to denote its resistance.

The vibrator is set into operation by closing the switch 35 which causes current to flow from the battery through the magnet coil 38, 15 thereby attracting the armature 36 of the vibrator into alignment with the upper face of the magnet core 31. This causes the reed 33 to move toward the right to bring the reed contact 32a into engagement with the stationary contact tact 4|. The engagement of contacts 32a and 32 causes a current to flow from the battery 20 through the transformer primary 24. The separation of the contact 42 from the contact 4| causes the magnet coil 38 to be disconnected from the battery, and its magnetism to diminish. However, the reed 33 continues to swing toward the right until its contact 32a is engaged with the contact 32. By this time, the magnetism of the coil 38 has diminished to zero thus permitting the reed by its own resiliency to swing back through its normal position and to the left of normal due to the momentum of reed and armature, thereby causing contact 32a to be separated agesoccurring alternately in opposite directions.

If unidirectional current is desired, the secondary voltages may be rectified either by a mechanical vibrator rectifier associatedvwith the primary interrupter or by means of a thermionic rectifier. The diagram shows a thermionic rectifying tube 50 having plates connected with the ends of the transformer secondaries'25 and 26, and a cathode 52 connected by a filter choke 53 with an output terminal 54. .The center tap of the transformer secondary is connected by line wire 55 with terminal 56. Filter condensers 51 and 58 connect the ends of choke 53 with output lead 55. The tube filament 59 is connected by wires: 60 and GI with switch 35 and battery 20.

I have thus far described a conventional form of D. C. 'o'nverter system employing a vibrator, the magnet coil of which is controlled by contacts in series therewith. Without somemeans of suppressing sparking at the vibrator contacts C, the life of the contacts and hence of the vibrator will be relatively short. Therefore some efiective means for eliminating sparking at these contacts must be supplied. I employ, as a spark suppressor, aresistance it! in shunt with the magnet coil 38, and I show by the following discussion how the requirements of spark elimination are met by a resistor of the proper value in shunt with the magnet coil winding. The letter 1' is also applied to the resistor fill to indicate its resistance.

In the discussion which follows I assume, first, that the vibrator system is the conventional one without the shunting resistance 70.

When the contacts C close the battery voltage E is impressed on magnet coi l 38. Voltage E is consumed by the-counter generated voltage due to induction and the iR. drop.

Let

'e=the counter generated volts due to induction.

i=the current in the coil at any instant after closing the contacts.

R=the resistance of the coil in ohms.

L=the inductance of the coil in henries.

Equation 3 shows that at any time aftef'clo'sin'g of the contacts the counter generated volts and z'R. drop in the coil depends on the value of the instantaneous current i.

t=0, i'R=0, ad 9:2

and when t= mag c, and 1R=E when I =the maximum steady current after the end of the transient.

The instantaneous current i at any time after closing the contacts may be determined as follows:

When t: 0, =0

In orderto reduce the sparking across contacts C to a negligible amount, the inductive voltage across contacts C must be keptat a value that will not cause sparking when the circuit is interrupted. D

The magnitude of the voltage across the -vibrator coil after the contacts open may be dewhere -8o=the inductive voltage across the coil and contacts when the contacts 'open.

. Difierentiating 6 gives:

E6 Equation 7 shows that regardless of the value of it) a when t=0, eo=E, where R is the resistance of the circuit when the contacts are open. If, however,.there were no resistance across the contact or across the coil at the time the contacts separate the resistance of the circuit would be infinity. Further examination of Equation 7 will show that ergy would be R+r ohms.

Substituting this in 'lgives:

Equation 8 shows that the lower the value of r is made the lower the voltage across the contacts will be at the time of opening.

Since R and r are in'parallel across the battery, 1 must not be made too low or an excessive current would be drawn and burn up the contacts. Therefore, the best value 01' r is the maximum value that may be used for the necessary reduction of sparking at the contacts. This will allow theuse of minimum size resistor and maximum electrical economy.

Investigation on a typical breaker and vibrator coil showed that 1' may be as high as twenty times R, although a value of about ten times R 7 would be better.

While the embodiment of the present invention as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted, all coming within the scope of the claim which follow.

What is claimed is as follows:

Asystem for converting low voltage direct current into higher voltage comprising a transformer having a divided primary, a source of direct current, a vibratory interrupter having a reed connected in series with the current source and center tap of the transformer primary, contacts respectively connected with the ends of the transformer primary and engageable alternately with contacts carried by the reed, an armature carried by the reed, an electromagnet cooperating with said armature and having a winding connected with the current source through normally closed contacts actuated by the reed, and means for reducing sparking at said contacts and comprising a non-inductive resistance in shunt with the magnet winding.

. RAYMOND H. SULLIVAN. 

